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Physical Society ColloquiumInterview for Faculty PositionCritical Field Enhancement near a Superconductor-Insulator Transition in a Triplet Quasi-1D SuperconductorI.J. LeePrinceton UniversityThe Bechgaard salts (TMTSF)2X (where X=PF6, ClO4 etc.) are about as exotic as it gets when it comes to electronic materials. They show spin density waves (SDW), quantum Hall effects associated with a field induced SDW, quantum oscillations, angular dependent ground states related to magic angle effects and unconventional superconductivity. Taking advantage of the remarkable anisotropic properties of these salts, I was able to obtain the upper critical field to 9 T (for a 1 K superconductor!) with near perfect alignment of sample to the applied field. The measured upper critical field exceeds the Pauli (spin) pair-breaking limit by more than a factor of four. This huge enhancement of Hc2 strongly suggests triplet superconductivity, which has been confirmed by Knight shift (Ks) measurements. The Ks experiments were done with simultaneous transport and pulsed NMR at dilution temperature, 1-2 Tesla of exactly aligned magnetic fields and 6-7 kbar of applied pressures.In addition to being a spin triplet superconductor, the nature of the strong suppression of orbital diamagnetic frustration is of great interest, since it may point a way toward high critical field materials. At low pressures this material is an insulator (SDW) while it is superconducting above a critical pressure, Pc~5 kbar. Well into the superconducting phase the critical field can be understood by Ginzburg-Landau theory. However, as pressure is reduced toward Pc, there is a strong enhancement in the critical field and a signature upward curvature in Hc2 vs T. The upper critical field is increased by ten fold at low temperature near Pc while the zero field critical field is remained the same at 1.2 K. I will discuss a slab/stripe model for the strong suppression of the orbital superconducting frustration, with alternating “slab” of superconductors and SDW phases. The new mechanism we propose, self-consistently dividing the superconductors into thin sheets separated by insulating layer, should be possible in any systems where the superconducting and insulating phases are nearly degenerate.
Monday, March 17th 2003, 16:00
Ernest Rutherford Physics Building, R.E. Bell Conference Room - room 103 |